1. Field of the Invention
The present invention relates generally to communication systems. More particularly, the present relates to a receiver for bandspread signals.
While the present invention is described herein with reference to a particular embodiment for a particular application, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope of the invention.
2. Description of the Related Art
In the field of signal processing, bandspread signals are signals in which a high frequency carrier is encoded with a pseudo-random code (hereinafter PRC). The PRC, which is a digital spectrum-spreading code, is modulated by a modulation signal and is employed for data transmission. The modulation signal is often also a digital signal. In such a case, the second digital signal is superimposed on the first digital signal (e.g. the PRC) for data transmission. Generally, the clock rates of the first digital signal (PRC) and the second digital (modulation signal) are significantly different.
The PRC is employed by the transmitter for bandspreading signals and is also stored in the receiver. Moreover, the receiver also contains a clock pulse generator for controlling the read-out of the PRC of memory. The phase of the clock signal must be controlled so that the PRC stored in the receiver is aligned with that of the received signal. The distance of the transmitter from the receiver can be determined by the phase shift between the received PRC and the stored PRC.
Systems employing bandspread signals are, for example, the JTIDS (Joint Tactical Information Display System) and the GPS (Global Positioning System). The GPS is described in "Navigation: Journal of the Institute of Navigation", Vol. 25, No. 2, Summer 1978, pages 121 to 146. The article is entitled "GPS Signal Structure and Performance Characteristics", by J. J. Spiker, Jr.. Receivers for bandspread signals are described on pages 139 to 146 of the above-cited GPS publication. The implementation of such receivers is very complicated and costly.
A receiver for modulated bandspread signals is also disclosed in the above-identified parent application. This receiver is easy to implement and suited for very large-scale integration. A number of functions can be performed by a computer. The RF section requires minimal circuitry. In particular, no frequency nor amplitude control means are necessary. The receiver can be easily expanded to permit communication with several satellites on a time-division multiplexed basis. This is necessary particularly if it is implemented as a GPS receiver. The received signal is digitized in such a way that the digital signal has a first or a second value depending on given criteria. Such a receiver is suitable for the C/A code of the GPS system.
The object of the present invention is to provide a receiver having the properties of the receiver disclosed in the parent application in such a way that it is also suitable for processing the P-code of a GPS system.
Another object of the present invention is to provide a receiver which is capable of processing both the C/A and P codes of a GPS system with lower losses.
These and other objects and advantages of the present invention will become apparent to those of ordinary skill in the art having access to the teachings provided herein.